Locations

Drops

Attacks

Their most used attack is a small lunging strike that is surprisingly powerful.
They also seem to have an attack in which they jump at you. If grabbed, the spider will then crawl behind you, and cut your throat, similar to the hollow thieves in the first Dark Souls backstab animation, except the spiders will immediately kill your character, even at a high Soul Level (only ever saw this from NG+, but definitely killed the shit out of me with full health and Third Dragon ring).

Strategy

Try not to get overwhelmed by their numbers. Avoid being surrounded and use attacks that cover a wide area to keep them stunned and not sucking out your bodily juices for sustenance.

I have never seen this backstab from a spider. Someone please post a video of it. If not I can't help but feel like this is some kind of long going prank in the dark souls 2 community (to make people believe it's real).